Subsequent Event - Additional Information (Detail) (Medco, USD $) In Millions, unless otherwise specified	1 Months Ended
Feb. 28, 2013
Medco
Subsequent Event [Line Items]
Upfront cash payment	$ 25.0
Payments made to the company for clinical development, regulatory approval and commercial milestones total	$ 180.0

Represents the maximum amount of pre-specified milestone payments that could potentially be received under the collaborative and licensing agreement.

No definition available.

Name:	alny_MaximumNumberOfPotentialFutureMilestones
Namespace Prefix:	alny_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration